Butachlor of analytical grade (97.7%) was obtained from Sigma-Aldrich (Germany). All solvents and chemicals were of A. R. grade.

The investigation was based on three simultaneous experiments in the laboratory of Department of Agricultural Chemistry and Soil Science, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal, India and the experiments were conducted with three soils from three different geographical origins possessing a wide variation in physicochemical properties (Table 1). Surface soil samples (0 - 10 cm), collected from the monoculture (kharif rice) cultivated fields of all three different locations, were kept in a green house at 25˚C ± 5˚C at 60% - 80% of maximum water holding capacity. Shortly before the study, soils were air-dried, ground and passed through a 80 mesh sieve, thoroughly homogenized and analyzed for pH (1:2.5 soil-water ratio) [29] ; organic carbon by 1 N K₂Cr₂O₇ solution method [30] ; total nitrogen by modified Kjeldahl method [29] and available P₂O₅ by extraction with alkaline NaHCO₃ (pH 8.5) method [31] . Mechanical analysis for determining the texture of the soil was done by Hydrometer method [32] [33] . Carbon-di-oxide evolved per 100 gm soil following the method of Pramer and Schimidt [34] by absorbing the evolved carbon-di-oxide in NaOH solution and back titrating with hydrochloric acid using Barium chloride and phenolphthalein as indicator.

Soil sample was drawn from the respective pots periodically (5^th, 10^th, 15^th, 30^th, 45^th and 60^th days after incubation) from the replicated pots of each treatment. The moist soil samples were analyzed to count the colony forming units (CFU) of total bacteria and fungi following serial dilution and pour plate technique [36] in asparagine-mannitol agar [37] and dextrose-rose bengal agar [38] , respectively. Soils (sterile and non-sterile) were also analyzed for the presence of butachlor by extracting the soils following the procedure described by Debnath et al., [39] . For residue analysis, soil samples (sterile and non-sterile) were also collected on “0” day (1 h) in addition to the periods stated above. Herbicide residues were analyzed by GLC [Hewlett Packard (USA), model 5890A] coupled with a Ni⁶³―electron capture detector, a glass column (6^/ × 2 mm) packed with 3% OV-101 on 80 - 100 mesh chromosorb-W (HP), and 3329A integrator. During analysis following gas chromatographic parameters were maintained: injection temperature, 250˚C; oven temperature, 200˚C; detector temperature, 250˚C; carrier gas, nitrogen having 65 mL/min flow rate. The residue data were processed to calculate the half-life (T_1/2) following the method of Hoskins [40] .

The results were evaluated by analysis of variance (ANOVA), and the statistical significance (P = 0.05) of difference between means within factors (herbicide, soil type and incubation time) was computed using Completely Randomized Design following the method of Gomez and Gomez [41] .

Sorption and degradation are two fundamental processes which govern predicting the environmental fate and behavior of organic contaminants in soil, which are affected by many factors like interaction with microorganisms, chemical and soil constituents [42] . A study on butachlor dissipation was carried out in soil in presence and absence of biological activity under laboratory condition. The dissipation and thus persistence of butachlor varied across soil types and prevailing conditions (sterilized and non-sterilized). After 60 days of incubation, 45.9% - 49.5% and 83.9% - 90.5% attenuation of butachlor residues were noticed in three soils under sterilized and non-sterilized condition, respectively (Table 2). When log residues in all cases were plotted against time, a linear relationship (r², 0.85 - 0.98) was observed suggesting a first order reaction in the dissipation behavior of the herbicide (Figure 1), which conforms earlier findings [43] [44] . The half-life (T_1/2) values were calculated by means of log 2/K, where K represents to the slope of the straight line regression. Under non-sterilized condition

^*Mean value of three replications.

rate of attenuation was highest in alluvial soil (T_1/2, 20.1 d) followed by lateritic soil (T_1/2, 21.5 d) and coastal (T_1/2, 25.1 d) soils. However, under sterilized soil the trend was as follows: alluvial soil (T_1/2, 75.3 d) = lateritic soil (T_1/2, 75.3 d) < coastal soil (T_1/2, 100.3 d). The half-life found in the present investigation under non-sterilized condition is quite comparable with the half-life values found earlier (T_1/2, 16 - 19 d, [45] ) under field study, using application rate similar to our present study (2 kg ai/ha). However, the observed minor difference might be due to the differences in experimental conditions in two studies. Opposite to our experiment they conducted the experiment in rice grown soil under field condition with and without organic manure treatment, which attributed to difference in physiological metabolic activities in soil. The biochemical activity is also encouraged by root exudates [46] . However, quality and quantity of root exudates depend on plant types and microorganisms also have a choice on root exudates utilization [47] . In a separate experiment half-life value of butachlor in the rhizosphere soil containing three different plants ranged from 5 - 10 d [28] , which also reflects the same explanation. Faster degradation of butachlor in alluvial than that of coastal soil, observed in the present study, was also previously reported [48] . The low persistence of the chemical in alluvial soil was earlier demonstrated due to the most congenial environmental factors particularly pH [44] , favoring microbial degradation process through enhanced activities of micro-organisms [49] . On the other hand, it was highly persistent in coastal soil due to higher salt content [50] adversely affecting growth and activities of micro-organisms as evidenced from the least microbial activity across soil type. Commendable dissipation was observed in sterile soil. This indicates the involvement of abiotic forces (sorption and chemical factors) other than photo degradation as the soils were kept under dark during the entire experimental period. In non-sterilized soil herbicide dissipation was higher than that in sterilized soil, confirming the possible microbial role in addition to abiotic factors. It is also interesting to note that irrespective of soil types and days of incubation there was a general trend of more dissipation of the chemical due to abiotic forces (49.5% loss after 60 d) than biotic forces (41.0% loss after 60 d) (Table 2). Sorption plays a vital and influencing role on organic contaminants fate and behavior in soil because the process exerts tremendous impact over the bioavailability of the chemical for degradation [51] . Adsorption of butachlor in soil is well established phenomenon and well documented [52] -[54] .

Substantiating the results of Min et al., [15] and Barman et al., [55] butachlor effectuated significant detrimental influence on carbon di-oxide production, the virtue of microbial activity, from the beginning up to 10, 13 and 22 d of incubation in coastal, lateritic and alluvial soil, respectively (Table 3). The initial detrimental influence reflected the lag phase or adaptive phase of microbial community for synthesizing required enzymes [56] to degrade the herbicide with the formation of new strains through mutation [57] and then multiply their population to the required level not only for the detoxication [26] but also the utilization of either herbicide [58] , the dead susceptible biotic agents [59] or both as energy and nutrient sources. Consequently, supporting the reports of Min et al., ( [15] there was a significant boost in microbial activity from 13^th, 16^th and 28^th day of incubation in coastal, lateritic and alluvial soil, respectively, persisting thereafter throughout. Among the three soil types, significant higher amount of carbon di-oxide was released from alluvial soil followed by lateritic and coastal soil, respectively. The lowest microbial activity in coastal soil was due to the hypertonic soil environment [60] while

the highest microbial activity in alluvial soil [61] was due to the most favorable environmental condition particularly the pH [44] . Consequently during a period of 63 d, the cumulative release of carbon di-oxide from alluvial soil was 33.6% higher than that of lateritic soil which, in turn was 22.3% higher than that of coastal soil.

Butachlor brought about differential influence on colonial micro-organisms at different stages of incubation which, in turn, varied in different soil types. The herbicide caused an initial non-significant influence on bacterial population in each soil type―10 d for lateritic as well as alluvial soils and 45 d for coastal soil (Figure 2). For lateritic and alluvial soil the non-significant influence reflected stationary phase. Whereas, significant stimulation of bacteria by application of butachlor on the 15^th day of incubation in spite of significant detrimental influence on microbial activity during initial 13 and 22 d of incubation in lateritic and alluvial soil, respectively, ensured that the bacterial flora derived their energy and nutrients from herbicide for their growth and multiplication [62] . Consequently the population of bacteria significantly increased in lateritic and alluvial soil by 22.7 and 12.4%, respectively over their respective control on the 15^th day. The stimulation was more pronounced in lateritic soil. That was due to the additive effect of energy and nutrient from the susceptible dead cells as evidenced from significant enhancement in microbial activity on the 16^th day of incubation. Then there was another stationery phase persisting up to 30^th day and 60^th day for lateritic and alluvial soil, respectively. Butachlor at the later stages in lateritic soil, however, induced significant detrimental influence on bacterial population manifesting the activation process through which a putative herbicide was converted to toxic molecules [26] . As a result, there was a progressive activation of butachlor resulting in significant gradual reduction in colonial bacteria from 16.3% on the 45^th day to 30.0% on the 60^th day of incubation as compared to that of untreated control in

lateritic soil. Butachlor also brought about significant reduction in colonial bacteria by 17.2% as compared to the untreated control in the coastal soil on the 60^th day of incubation. Sustaining the reports of Min et al., [63] fungal propagules were significantly increased on the 5^th day of incubation by the application of butachlor over that of untreated control in lateritic soil illustrating the utilization of the chemical as nutrient and energy sources by the chemo heterotrophs (Figure 3). Then there was manifestation of the stationery phase on the 10^th day before the

influence of activated compounds. Thereafter, there was a significant progressive devastating impact of activated herbicide molecules resulting in sharp decline in fungal propagules from 40% on the 15^th day to 64% on the 45^th day of incubation. But on the 60^th day there was again detoxification process as evidenced from the non-significant influence in the lateritic soil. In contrast, butachlor resulted in the stationary phase of fungal propagules on the 5^th day in coastal soil before induction of activated compounds. Then activated compounds of butachlor resulted in significant reduction in fungal propagules from 46.4% on the 10^th day to 37.5% on the 15^th day revealing a significant gradual declining impact of activated compounds of butachlor and/or butachlor as such, which were detoxified thereafter from 30^th to 60^th day of incubation as evidenced from their non-signifi- cant influence. This conformed earlier finding [62] . Whereas, butachlor resulted in significant reduction in fungal propagules in alluvial soil as compared to untreated control on the 5^th day of incubation. The impact was more severe as evidenced from the 40% reduction in fungal propagules on the 10^th day. Following a mysterious non-significant influence on the 15^th day the activated butachlor molecules again resulted in a significant fall in fungal propagules by 32.1% as compared to untreated control on the 30^th day. Thereafter, butachlor resulted in a significant enhancement in fungal propagules during later stages of incubation in alluvial soil, commemorating the findings of Yang et al., [28] . The extent of increment, however, gradually decreased from 75% on the 45^th day to 45.5% on the 60^th day as compared to untreated control in alluvial soil. So far as soil types are concerned, alluvial soil, owing to most favorable environmental conditions particularly the pH, was the best niche for the maximum proliferation of colonial bacteria followed by lateritic and coastal soils, respectively. On the other hand, lateritic soil due to favorable acidic pH harbored the highest fungal propagules followed by alluvial and coastal soil, respectively. Coastal soil reared the least colonial bacteria and fungi due to higher osmotic soil environment. The results substantiate the findings of [14] . Butachlor was subjected to co-metabolism by commensal microbes during initial 10 days of incubation and 19.5% residues were dissipated as evidenced from non- significant influence of herbicide on colonial microbes in alluvial soil (Figure 2 & Figure 3, Table 2). Then the significant increment in colonial bacteria on the 15^th day pointed out that 13.3% butachlor residues were utilized by zymogenous [64] bacteria as energy and nutrient sources besides commensal microorganisms from the 10^th to 15^th day of incubation. Again there was 1.8% of residual loss in butachlor through co-metabolism from the 15^th to 30^th days. But during later stages, 6.4% of butachlor residues were utilized by colonial fungi in alluvial soil along with co-metabolic micro-organisms. On the other hand, co-metabolism was the only phenomenon prevailing in coastal soil and 38.0% of butachlor residues were co-metabolized in coastal soil by commensal microbes (Figure 2 & Figure 3, Table 2). However, along with co-metabolic microbes, colonial fungi were also the responsible agents during initial 5 d in lateritic soil and 12.9% of butachlor residues were utilized by them as nutrients and energy sources as evidenced from significant increment in fungal propagules. Then there was co-metabolism of 5.3% of butachlor residues from 5 to 10 d of incubation. But again from the 10^th to 15^th days of incubation about 5.7% of butachlor residues were metabolized by zymogenous bacteria together with co-me- tabolic ones as evidenced from their significant stimulation. Thereafter, from 30^th to 60^th day about 10.4% of butachlor was co-metabolized in soil.